1. Field of the Invention
The present invention relates to an improved modulation method and apparatus providing phase and frequency modulation for generating the narrow band signals required for many communications systems, specifically those of multichannel mobile and fixed base systems using low and high VHF bands and UHF bands. The invention uses both a direct synthesized frequency source and a narrow band FM modulator.
The improved modulation method and apparatus according to the invention provides modulation of up to 5.0 KHz peak deviation in response to modulating signals from approximately 60 Hz to 3.5 KHz. Typical systems require pre-emphasis using conventional 750 .mu. sec delays to maintain optimum modulation levels. Distortion levels of less than 1.0% are desirable with frequency response performance less than 6 db over an audio frequency range of 300 Hz to 3.0 KHz.
2. Brief Description of the Prior Art
Frequency synthesizers for radio applications and frequency shift key modulators for communication apparatus are well-known in the patented prior art as evidenced by the U.S. patents to Nossen U.S. Pat. No. 4,114,110 and Otto U.S. Pat. No. 4,068,198, respectively. The Nossen patent for example discloses a frequency synthesizer employing dual phase lock loops for rapid switching and fine resolution. The Otto patent discloses a phase locked loop frequency shift key modulator wherein the loop includes a programmable divider and wherein the output of the oscillator is also divided to obtain the desired carrier frequency.
As will be developed in greater detail below, while the prior devices normally operate satisfactorily, they each possess inherent drawbacks including limited frequency range, degradation of the modulating signal, an undesirable signal to noise ratio, and impracticality owing to increased complexity and cost.
The present invention was developed in order to overcome these and other drawbacks of the prior devices by providing a modulation method and apparatus which is simple to manufacture and suffers from no mechanical perturbations. The phase locked loop parameters of the invention may be selected for optimum system spectral response and modulation is independent of the desired transmit frequencies.